This invention relates to a piezoelectric ceramic composition for an actuator and, in particular, to a piezoelectric ceramic composition that is extremely useful for piezoelectric actuators because of its higher piezoelectric strain constant, higher Curie point and lower relative dielectric constant.
Conventional design for some actuators uses an inverse piezoelectric effect that converts electric energy to mechanical energy. Actuators so designed are capable of producing fine movement in a precisely controlled manner in response to the application of voltage. Displacements of one micron or less are possible. This has contributed to the widespread usage of piezoelectric actuators in recent years. Such uses include precisely controlling acoustics or flow rate in buzzers, pumps and valves; auto-tracking and autofocusing in magnetic heads of video tape recorders, and precisely positioning mechanical cutting tools with the available maximum movements of less than one micron. Such actuators are also applicable to fine positioning devices for use in manufacturing semiconductors.
Lead zirconate titanate ceramics (called PZT) are known to exhibit superior piezoelectric properties when used as the piezoelectric material for actuators. PZT has been improved in various ways for individual applications.
The features and properties of piezoelectric materials used in PZT actuators have been improved by replacing a part of the lead zirconate titanate with trivalent ions such as Bi.sup.3+, or by synthesizing solid solutions of a composite perovskite compound and a compound such as Pb(Ni.sub.1/2 W.sub.1/2)O.sub.3, Pb(CO.sub.1/3 Nb.sub.2/3)O.sub.3 or Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3.
Piezoelectric actuator elements for controlling fine displacement of approximately one micron or less may be any of three types: unimorphs, bimorphs and laminated. Such elements are required to have, as the inherent properties thereof, a higher piezoelectric strain constant as well as a higher Curie point. In other words, the piezoelectric strain constant d.sub.31 of transversal mode and the Curie point Tc should be within the following values: EQU d.sub.31 &gt;300.times.10.sup.-12 m/v, Tc&gt;150.degree. C., (1)
or EQU d.sub.31 &gt;250.times.10.sup.-12 m/v, Tc&gt;250.degree. C. (2)
Materials having a piezoelectric strain constant d.sub.31 of higher than 300.times.10.sup.-12 m/v have a reduced Curie point (Tc) of around 100.degree. C. Accordingly, the operating temperature of an element made from such materials has an upper limitation of 50.degree. to 60.degree. C., restricting the practical applications thereof. In addition, such actuator elements are disadvantageous in that the relative dielectric constant .epsilon..sub.33 T/.epsilon..sub.0 is higher than 5,000 and the impedance thereof is thus likely to be reduced so as to cause the elements to heat when operated at a high frequency. Besides, the actuator element can only be operated with a driving power source having a large capacity. In addition, the use of ceramic materials having such a high piezoelectric strain constant d.sub.31 places various practical restrictions on the use of actuator elements made therefrom because the relative dielectric constant and the piezoelectric strain constant of such materials are generally highly dependent on the temperature.
On the other hand, ceramic materials having a higher Curie point (Tc) are typically low in their piezoelectric strain constant (d.sub.31). This means that an actuator element made of such material can only be operated with a high voltage.